What does the atomic number of an element refer to?

Match the mixture with the substance// i really need on this guys (it’s a pic btw)

Use this information to determine the number of calends electrons in the atoms. which of the following correctly compares the stability of the two atoms? a) both are unreactive b) both are highly reactive c) a is unreactive and d is reactive d) a is reactive and d is unreactive

What substance has a mass of 9.5g and volume of 2.1cm^3